Superfund Research Program
Bioaccumulation and Biomagnification of Metals in Lakes
Recent developments in the study of metal contamination in lakes have far reaching consequences that scientists are seeking to understand and to convey to the public. One issue of significance is that, even in what appear to be pristine waters, some metals are reaching surprisingly high levels in fish. Metals accumulate in the bodies of animals through the foods they eat. Some metals even increase in concentration as they pass through two or more trophic levels in a process known as biomagnification. Because of this cumulative build-up of toxins in aquatic food chains, humans who consume fish from lakes where bioaccumulation occurs may be exposed to levels of metals associated with adverse human health effects. Other animals at the top of aquatic food chains, such as fish-eating wildlife, are also highly susceptible to metal contamination.
Scientists have made significant advances in understanding the importance of food web relations to metal uptake in fish. This progress has led to scientific inquiry on the processes underlying the observed geographic patterns in metal contamination across lakes. For instance, scientists are now asking why metal concentrations in lake fish are not always correlated with contaminant levels in lake water or sediments, and why metals vary with respect to their potential for bioaccumulation or magnification. To better understand the reasons for variation among lakes, researchers at Dartmouth College are investigating the potential for metal transfer through aquatic food webs. They are especially interested in understanding the mechanisms of metal transfer across trophic levels and developing novel molecular markers for detecting metal stress in natural populations.
To date, over 20 lakes in the Northeastern United States have been studied. Baseline data on metal levels have been collected in water, particulates, sediments, plankton and fish. The samples are being analyzed for mercury, cadmium, lead, arsenic and zinc using advanced high resolution technology being developed by scientists in the Dartmouth Superfund Basic Research Program Trace Metal Core Facility.
So far, their research has revealed that the concentrations of metals in fish are related to the overall structure of aquatic food webs. Food webs with longer food chain lengths (i.e., more possible steps from plant to animal) and fewer linkages (i.e., fewer possible connections between predators and prey) have higher concentrations of metals, such as mercury and zinc, in fish. They also find that certain groups of freshwater invertebrates (e.g., cladocerans) accumulate higher concentrations of metal than other groups (e.g., copepods). These fundamental processes and species patterns help to explain the underlying variation in metal uptake observed in lakes that have different food webs.
In a closely related study, the Dartmouth scientists are investigating methods for the early detection of metal stress in aquatic organisms. The goal is to develop a rapid and informative biomarker that will serve as an early warning system for lakes in which aquatic fauna are exposed to stressful levels of metals. Specifically, they are investigating biochemical indicators of metal stress in zooplankton, such as induction of heat shock proteins and metallothionein protein, and exploring their use as field biomarkers. To date, they have characterized two heat shock proteins in Daphnia pulex, the common water flea, which respond to sustained arsenic exposure. They have also documented that Daphnia pulex responds differently to the various chemical forms of arsenic and that response varies between ages and classes of Daphnia. When fully developed these biomarkers will allow scientists to identify lakes in which low levels of metal contamination in the water may be adversely affecting organisms.
The development of these methods, along with the study of the underlying mechanisms of metal transfer in aquatic food webs, are important for identifying lakes with the greatest potential for concentrating metals in food chains. Because metals can be toxic at high concentrations, this knowledge is crucial for preventing human exposures to harmful levels of metals that may be present in lake fish. This research is also important for understanding the effects these processes have on the natural functioning of ecosystems to include the functions that may be impaired and the organisms that may be lost as a result of metal toxicity.
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To learn more about this research, please refer to the following sources:
- Folt CL, Chen CY, Moore MV, Burnaford J. 1999. Synergism and antagonism among multiple stressors. Limnology and Oceanography 44(3):864-877.
- Stemberger RS, Chen CY. 1998. Fish tissue metals and zooplankton assemblages of northeastern U.S. lakes. Canadian Journal of Fisheries and Aquatic Sciences 55:339-352.
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